1. Field of The Invention
The present invention relates generally to systems for controlling the elevation above ground level of a tool of a road construction machine, and more particularly, but not by way of limitation, to a grade averaging system for a pavement milling machine.
2. Description of the Prior Art
It is common practice in the design of various construction apparatus, such as pavement milling machines, paving machines, curb forming machines, and the like, to provide an automatic elevation control system. The automatic elevation control system will typically measure a distance between a sensor mounted on the machine and a reference point fixed relative to the ground. The reference point may in fact be the ground surface, or it may be a guide string or the like.
The sensing system may be of the type which engages the referenced surface or it may be of a non-contact type. Sensing systems which engage the referenced surface include those using skis or rollers which traverse the ground surface, and those which include a sensor arm which engages the guide string or the like.
Those sensing systems which use non-contact sensors may use ultra sonic sensors, laser sensors, or the like.
One commonly used prior art elevation control system is that sold by Moba Electronic Gesellschaft fur Mobil-Automation MbH, as is described in detail in U.S. Pat. No. 5,309,407 to Sehr, et al., the details of which are incorporated herein by reference. The Sehr, et al., system is designed for use with a milling machine which has its cutting drum mounted to a mid portion of the frame, approximately midway between its forward and rearward traveling gears. As used herein, the term "traveling gears" includes any of the conventional ground engaging means used on construction equipment, typically either wheels with rubber tires or an endless track system.
In the Sehr, et al., system, three ultra sonic elevation sensors are utilized. All three sensors are fixably mounted on the frame of the machine. One sensor is located approximately at the position of the forward traveling gear, a second sensor is located approximately adjacent the cutting drum, and the third sensor is located approximately adjacent the rear traveling gear. The three sensors are preferably equi-distantly spaced. The computerized control system receives signals from each of the three sensors representative of the distance between that sensor and the reference surface. The control system averages those three signals, and uses this average signal to control the cutting depth of the cutting drum. By using an average signal taken from three different sensors, this system minimizes the undesired effect of irregularities in the road. As will be appreciated by those skilled in the art, if only a single sensor were utilized, it would try to exactly reproduce irregularities which it senses in the roadway.
It will be appreciated that with the Sehr, et al., system wherein all three sensors are fixedly attached to the frame of the machine, if one of the wheels encounters an anomaly, e.g. a hole, in the road, that anomaly will not be sensed by the sensor adjacent that traveling gear if the sensor also is located above the hole. Instead the effect will be for a portion of the anomaly to be sensed at the middle sensor adjacent the cutting drum. The sensor at the other traveling gear would also not sense any anomaly.
This system works adequately with a machine like that illustrated in Sehr, et al., for which it was designed, namely one with the cutting drum located midway between the forward and rearward traveling gears. This permits three sensors to be located approximately adjacent the forward and rear traveling gears and the cutting drum, and the average elevations sensed by those sensors is utilized to control the cutting drum in the manner just described.
There are, however, other types of commonly used construction equipment for which the system of Sehr, et al., is not operable. For example, another common design of roadway milling machine is that sold by the assignee of the present invention as the Wirtgen Model W1000 Cold Milling Machine. The Wirtgen Model W1000 machine has a pair of forward wheels and a pair of rearward wheels. The rotating cutting drum is located directly between the pair of rearward wheels. Thus, there is no longitudinal separation between the cutting drum and the rearward wheels.
The inapplicability of the Sehr, et al., system to use on a machine like the Wirtgen Model W1000 led the inventor to develop a substantially modified system which would be useable with a machine wherein the cutting drum is located directly between a pair of wheels. The result has been a grade averaging system which is much more universal in its applicability. As will be shown, the system of the present invention can be used with a machine geometry like that of the Wirtgen Model W1000 machine, and also can in fact be used with improved results upon machines having a geometry like that of the Sehr, et al., machine.